In alternating current track circuits for electrified railroads using alternating current propulsion power, the track relay means on occasion may be subject to application of propulsion power at a high voltage level. This may be due to any one of several conditions, for example, the opening of an impedance bond connection or a broken rail in the section. Although the signaling and propulsion sources have different frequencies, with the propulsion power normally at the lower frequency, harmonics of the propulsion frequency may be at or near the frequency of the signaling current to which the filters and/or the track relay inputs are tuned. High voltage interference from the power source then can result in the track relay being energized to register an unoccupied track section even though a train may actually be occupying the corresponding section. Obviously, this is an unsafe, even dangerous condition and where occurrence is likely, some arrangement to protect against this fault condition is not only desirable but may be required.
Accordingly, an object of my invention is a coupling arrangement between an alternating current track circuit receiver and the rails in the section to vitally protect against high voltage surges or spikes induced into the rails.
Another object of the invention is a vital protection arrangement against interference in a railroad track circuit by harmonic frequency components of the propulsion currents.
A further object of the invention is a vital parallel resonant transformer apparatus used in coupling an alternating current track circuit receiver network to the section rails to inhibit activation of that receiver by currents having harmonic frequency components of the propulsion power.
A still further object of the invention is a vital parallel resonant transformer unit whose primary winding is coupled to the track rails, a secondary winding is connected to the track circuit receiver, and a third winding, capacitor tuned to the track circuit frequency, in combination with the primary prevents propulsion power interference with track circuit operation.
Yet another object of the invention is vital protection apparatus for an alternating current railroad track circuit which includes a transformer having a primary winding connected in series with a fuse having a preselected capacity and coupled to the rails through a track transformer, winding connected to the track circuit receiver input, and a third winding tuned by a capacitor to resonate with the primary at the track circuit frequency but presenting a low impedance to propulsion current so that high levels of this current will interrupt the track circuit receiver connections to the rails by opening the fuse.
Other objects, features, and advantages of the invention will become apparent from a following specification and appended claims when taken in connection with the accompanying drawings.